A few nights ago, Carlos Acosta returned to Havana's ornate Gran Teatro Garcia Lorca to dance beneath a frieze of naked women and bring a great audience to its feet. The star arrived with two fellow performers from the Royal Ballet, Leane Benjamin and Ricardo Cervera, and also a small slice of Mayerling, a Kenneth MacMillan ballet that deals in drugs, misery and self-harm. It's not what the Cubans, fed on a diet of perfect Giselles and Swan Lakes, is used to.

This being one of the highlights of Havana's biennial International Festival of Ballet, the house was packed - the stalls and no fewer than five floors of balcony so alive with people that aisles and walls disappeared from view.

The Cubans were softened up first with a curiously lacklustre Les Sylphides performed by the usually perfect National Ballet of Cuba (there are many brutal years of ballet school behind their work, which few dancers from richer countries would tolerate).

But then the scene changed and there, suddenly, was Acosta, the hard man of ballet, his hair slicked back. They say ballerinas love Acosta because he never drops them, and Benjamin must have been grateful for that ability. In MacMillan's piece she gets thrown around and around, over his shoulders and under his legs. Understanding his audience's preference for chutzpah over subtlety, Acosta gave them the full works.

And they went mad in return, leaping to their feet a few minutes in and refusing to sit back down. Above, in the grand balcony, was Alicia Alonso, the 86-year-old prima ballerina assoluta of Cuban ballet, who looks gravely on any dancer who leaves her company, irrespective of whether it's for the chance to perform contemporary work abroad or simply for more money. She was impossible to read, sitting there throughout MacMillan's depiction of sex and drugs with a waxy and fixed smile.

Then came celebrated Swedish choreographer and dancer Mats Ek, who had flown in with Ana Laguna to perform Memory, a piece of his that deals with the elderly. The contrast with Acosta was fascinating, because here were two famous male dancers who can draw the eye with equal power, who have a stage presence that is naturally and instantly gripping, but who are completely opposed in style.

The audience may have loved the Mayerling, but what they were really waiting for was the appearance of two national heroes together, Acosta performing with one of the resident stars of Cuban ballet, the beautiful Viengsay Valdes. Despite her demure, smiling manner, Valdes is Acosta's match in bravura. Her name means "victory" in Laotian (her father was the Cuban ambassador there), and the same goes for her performances. For her, 32 fouettés is a breeze.

Then, on Monday, she went down with a fever, and with it an asthma attack. She cancelled an outdoor appearance in Don Quixote as a storm moved in over Havana. She cancelled everything, in fact, but this one dance, the pas de deux of Diana and Actaeon.

Acosta arrived ready for what was supposed to be a joy-filled competition of athleticism and grace. His arrival on stage had the audience bellowing. He had made up a new, incredibly dangerous, move to entertain them - a barrel turn to which he had attached a mid-air fouetté, in which, essentially, he swivels round while hovering upside-down. A single misjudgment and he would have smashed a knee.

Unfortunately, however, Valdes wasn't on form. She retained her grace and smile but slipped - all but swooned - from her pirouettes and fouettés. On several occasions she looked about to fall over. Fever induces dizziness and it is hard to imagine why she would have wanted to do this, except for one obvious and heartbreaking reason: she must have been looking forward to dancing with Acosta for so long that she couldn't bear to lose the chance.

It was both brave and terrible; although she survived the dance, she was, apparently, taken away by ambulance. The audience had heard rumour of the illness and cheered for her anyway.